Photoelectric weft detector

ABSTRACT

A photoelectric weft detector for fly-shuttle weaving looms utilizing photoelectric sensing means comprises a first photocell positioned to sense specular and diffuse reflected light from a specified surface area of a bobbin, and a second photocell positioned at approximately the same distance from the specified area of the bobbin to receive only diffuse reflected light. The photocells provide a voltage proportional to their resistance ratio for activating a mechanism which replaces the bobbin upon exhaustion of the weft material.

O United States Patent [151 3,693,671 Desai 1 Sept. 26, 1972 [54] PHOTOELECTRIC WEFT DETECTOR FOREIGN PATENTS OR APPLICATIONS [72] lnventorI Dhimfll Deni, Palatine, 1,411,171 8/1965 France ..139/273 A [73] Assrgnee: 5h; Singer Company, New York, Primmy Examiner flemy S Jaudon Attorney-Marshall J. Breen, Chester A. Williams, Jr. [22] Filed: March 12, 1971 and Martin Sachs [21] Appl. No.: 123,634 ABSTRACT [52] U S Cl 139/273 A A photoelectric weft detector for fly-shuttle weaving [51] "5 45/12 looms utilizing photoelectric sensing means comprises [58] Fieid A 341 a first photocell positioned to sense specular and difl3'9/37O fuse reflected light from a specified surface area of a bobbin, and a second photocell positioned at approximately the same distance from the specified area of [56] References Cited the bobbin to receive only diffuse reflected light. The

UNITED STATES ATEN photocells provide a voltage proportional to their resistance ratio for activating a mechanism which g i et replaces the bobbin upon exhaustion of the weft 0 me e a m t L 3,459,240 8/1969 Erickson ..139/273 A a ma 3,440,634 4/1969 Maurmann et al. 139/370 8 Claims, 2 Drawing Figures PATENTEDsarzmsrz 3.693.671

' SHEEI 1 [1F 2 INVENTOR. Dhlmut R. Desol wmvsss:

NW4- mm "11.121" ATTORNEY BACKGROUND OF THE INVENTION The present invention relates to weft detection systems and more particularly to a photoelectric detection system for detecting the exhaustion of the weft material on a bobbin used in conjunction with a flyshuttle weaving loom.

Heretofore, weft detecting systems utilized mechanical sensing devices which were insensitive and wasted much of the weft material (yarn). These systems frequently frayed or tore the yarn due to the pressure exerted by the mechanical sensing device. Prior art photoelectric sensing devices were also insensitive and frequently could not sufficiently distinguish between an empty or full bobbin in high ambient light. Frequently the bobbin was modified by providing a light reflecting surface or by providing two special surfaces, one highly reflective and the other non-reflective to increase the sensitivity and reliability of the sensing device. However, since the bobbin was in constant use, the reflecting surfaces soon acquired different reflecting properties necessitating continual adjustment of the sensor. Bobbins, transparent with transmitted light, were also used in an attempt to improve the sensitivity and reliability of the detectors. Modification or replacement of the existing bobbins in order to be able to utilize any of the prior art weft detection systems is a costly added expense.

SUMMARY OF THE INVENTION A preferred embodiment of the present invention does not require any modification of the bobbins presently in use, is reliably operable in high ambient light, has a relatively high degree of sensitivity, and relies upon the inherent properties of the bobbins and weft material themselves for operation.

A system for detecting, in a specified area, the presence or absence of material having a diffuse reflecting surface and utilizing the principles of the present invention comprises, means having a specular reflecting surface for supporting the material, means for providing light to a specified area of the surface, first means arranged to sense the light reflected from the specified area in the presence or absence of material, the first means having an impedance proportional to the amount of light sensed, second means arranged only to sense lightreflected from the specified area in the presence of the material, the second means having an impedance proportional to the amount of light sensed, utilization means, and third means coupled between the first and second means and the utilization means for activating the utilization means in accordance with a predetermined ratio of the impedances of the first and second means responsively to the presence or absence of the material.

A complete understanding of the present invention may be obtained from the following detailed description, when taken in conjunction with the accompanying drawings in which:

DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of one embodiment of the present invention; and

FIG. 2 is a schematic circuit diagram of a preferred embodiment of a photoelectric weft detector.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings, and more specifically to FIG. 1, wherein the numeral 10 represents a photoelectric weft detector system suitable for mounting on the main frame of a conventional fly-shuttle weaving loom having an automatic bobbin replacement mechanism. The operation of the loom is in no way a1- tered by the present invention except that the automatic bobbin replacement mechanism is activated at the proper point of the loom cycle by a voltage obtained from a sensor utilizing the principles of the present invention.

A detailed explanation of the operation of a conventional fly-shuttle weaving loom is found in US. Pat. No. 3,373,773 granted Mar. 19, 1968 to George H. Balentine Jr., Richard W. Johnson, and Thomas L. Cox.

A holding frame assembly 12 of a preferred embodiment is made up of several sub-assemblies. A control box 14 houses the electronic circuitry required and has attached thereto a mounting bracket 16 and a base frame 18 used for mounting the holding frame assembly 12 to the main frame of the loom (not shown) in juxtaposition to a shuttle box 20 which is located on each end of the lay bar 22. An arm 24 which is affixed to the control box 14 is connected to an optical sensing head 26 and centrally positions the sensing head above the shuttle box 20.

Recessedly mounted within the optical sensing head 26 are two photoelectric sensing devices (photocells) 28, 30 and a light source (lamp) 32. Electrical connections between the photoelectric cells 28, 30, the lamp 32, and the control box 14 are made through channels in the arm 24.

Although the preferred embodiment of the present invention utilizes separate sections which are connected together to form the holding frame assembly it is to be understood that a one piece housing or assembly e.g., a casting, spot welded assembly, etc. may also be utilized.

A shuttle 34 having a bobbin 36 removably mounted therein is provided with an opening 38 through which the bobbin is visible as viewed from the optical sensing head 26. When the shuttle flys along the lay bar 22 and reaches the shuttle box 20 the stop 39 and the guides 40 and 42 position the shuttle 34 so that the opening 38 always occurs in relatively the same horizontal position.

The light rays emanating from a lamp 32 are focused by a lens 44 at a point 46 above the surface 48 of the bobbin 36. This results in an oval-shaped light projection on the surface 48 of the bobbin 36 approximately one-half inch long. The bobbin having a relatively smooth (specular) surface reflects sufficient light to photocell 28 to change its impedance but reflects relatively little light to photocell 30 and does not significantly change its impedance. If yarn threads 50 (weft material) are on the bobbin in the area 52, then the surface of the illuminated area will no longer be smooth and the light will be reflected at a multiplicity of angles because of the diffuse reflecting characteristic of the weft material. Relatively equal amounts of light will reach photocell 28 and 30 thereby changing their impedances approximately the same amount. The importance of the impedance changes of the photocells will become apparent when the operation of the detector is described in conjunction with FIG. 2.

It is to be noted at this time that the sensing of the weft on the bobbin is performed when the lay bar is in its most forward position, and that since the photocells are recessed within the optical sensing head 26 they substantially receive light rays only when the bobbin surface is in a specified position plus or minus small tolerance variations. This avoids a possible erroneous signal which might be obtained from the'surface of the lay bar which passes the detecting sensors before the bobbin comes to the most forward position. If the photocells were not recessedly mounted within the optical sensor head, the additional interlock or gating signal would be required to ensure sensing at the proper portion of the loom cycle to avoid erroneous readings. This may be accomplished by including an additional photocell and light source to sense the most forward point of travel of the lay bar; e.g., by an interrupter bracket breaking a light beam mounted on the main loom frame.

Incorporated within the control box 14 of FIG. 1 are the components, shown schematically in FIG. 2, that provide the DC voltage which in turn energizes a triggering solenoid 60 thereby ejecting the bobbin and starting the bobbin replacement cycle of the loom.

A source of power for operation of a preferred embodiment of the photoelectric weft detector 10 is the existing loom power supply 62 shown in FIG. 2, which is normally 5060 Hz at 10 to 15 volts. The 50-60 Hz is rectified by a full wave bridge circuit 63 including diodes 64, 66, 68, and 70. A fuse 72 is also included for overload and short circuit protection.

The unfiltered full wave DC voltage obtained from the bridge circuit is coupled to a regulated lamp supply circuit 73, which contains a resistor 74 and a Zener diode 76 to limit the voltage at a peak value of approximately 9.1 volts. For consistent sensing, the light intensity is maintained at a constant value by regulating the voltage to a light source 77 which in the preferred embodiment is a GB. No. 253 instrument type lamp operating at 2.5 volts and 0.35 amperes.

A voltage divider composed of resistors 78, 80 and 82 is connected across the rectified DC voltage source lines 83 and 85 and provides a reference voltage for the gate electrode 86 of the programable unijunction transistor (PUT) 88. Resistor 84 is connected in parallel with resistor 80 and is selected to adjust the voltage appearing at the gate electrode 86 of the PUT and is chosen to provide 2.5 volts rms across the light source 77.

At the start of a cycle a capacitor 90 connected from the anode electrode 92 of the PUT 88 to line 85 starts to charge through resistors 74 and 95 causing the voltage at the anode electrode 92 to increase at a rate determined by the RC time'constant [(R- R X C However, the voltage at the gate electrode 86 of the PUT 88 rises in accordance with the line voltage. The PUT is unable to fire until some time after 90 of a line frequency cycle has elapsed since the anode 92 cannot become more positive than the gate electrode 86 until the DC supply voltage decreases (between 90 and 180 of the line cycle). The anode electrode 92 tends to remain at a more positive voltage than the gate electrode 86 because capacitor 90 hold its voltage while the gate voltage decreases with the line voltage.

At a point in the cycle between 90 and 180 when the anode electrode voltage exceeds the gate electrode voltage by approximately 0.5 volt the PUT 88 fires (becomes low resistance between its anode 92 and cathode 96) thereby discharging capacitor 90 into cathode resistor 98 and the gate-cathode junction of silicon controlled rectifier (SCR) 100.

The anode-cathode junction of SCR 100 is connected in series with light source 77 across the source of DC voltage (lines 73 and When capacitor discharges into the gate-cathode junction of SCR it causes SCR 100 to fire thereby causing the DC source voltage to appear across the light source 77. Resistor 84 is chosen to set the firing point of the PUT at a specified portion of each cycle to obtain 2.5 volts rms across the light source 77 as mentioned earlier.

When the line voltage decreases, the voltage at the gate electrode 86 of the PUT 88 correspondingly reduces therefore the anode electrode 92 of the PUT 88 can become more positive than the gate electrode 86 earlier in the cycle thus increasing the conduction angle of the SCR 100. When the line voltage increases, the gate electrode voltage increases thereby making the anode electrode become more positive than the gate electrode at a later time in the cycle and decreasing the conduction time of the SCR. This circuit therefore provides a substantially constant voltage across the light source 77 for a varying input voltage source.

The second half of the line voltage cycle between 180 and 360 performs just as the first half previously described since the AC is full wave rectified. The SCR 100 resets (becomes a high impedance) each time the voltage goes to zero and the SCR has less than holding current flowing through it. The PUT 88 returns to its non-conducting state at substantially the same time since capacitor 90 is essentially completely discharged and starts its charging cycle as soon as the zero voltage portion of the cycle is passed.

Noise pulses (sudden reduction in voltage caused by the power SCR 104 firing) can cause premature firing of PUT 88 resulting in voltage surges across and possible burns out of light source 77 These noise pulses are prevented from reaching the gate electrode 86 by resistor 78 and capacitor 106. An additional light source 102 may be used as described previously and is connected in parallel with the light source 77 The full wave rectified DC voltage is also coupled to the DC regulator circuitry 108 via diodes 110, 112 and resistor 1 14. A filter capacitor 116 is coupled from the positive voltage line 118 to the reference (negative) line 121.

Diodes and 112 function to isolate the control circuitry from the regulated lamp supply circuit 73 used to feed the light source 77 and resistor 114 is a current surge limiter for the capacitor 1 16.

The DC voltage is regulated at approximately 9.1 volts DC in a conventional power supply circuit comprised of a transistor 119, a resistor 120, and a Zener diode 122. The regulated DC output voltage appearing between lines 124 (positive) and 126 (negative) is coupled to the photocells 28, 30 and 132; a comparator circuit 134; and a multivibrator circuit 136.

Resistors 138 and 140 are serially connected across the filtered and regulated DC voltage appearing across lines 124 and 126 and function as a voltage divider with relatively low resistance values as compared to the nonilluminated resistance values of photocells 28, 30, and

132. Their function therefore is to maintain the voltage at the anode electrode of PUT 142 at a predetermined value (50 percent of the voltage appearing across lines 124 and 126) should the photocells 28, 30 and 132 receive small differences of light due to ambient light changes or should the photocells open up or be removed for any reason.

The voltage appearing at the anode electrode PUT 142 would therefore depend on the resistance values of photocells 28, 30 and 132.

Photocell 132 is used if it becomes necessary to synchronize or interlock the functioning of the weft detector with a specific point in the loom cycle as mentioned earlier. This might be required if the photocells 28 and 30 were not recessed in the sensing head 26 (FIG. 1) and they were able to sense light reflecting from other than the specified surface area on the bobbin. When a light source (lamp) 102 illuminates photocell 132, photocell 132 changes to a relatively low resistance value. The voltage at the anode electrode of PUT 142 is unable to build up beyond the threshold value and no trigger pulse is obtained. Provision must be made to interrupt the light beam at the maximum forward position of the lay bar so that photocell 132 will return to a relatively high resistance value at that time. Then the ratio of the resistances of photocells 28 and 30 will determine the voltage at the anode electrode of PUT 142.

A capacitor 144 is coupled from the anode of PUT 142 to the negative line 126 to cooperate with resistor 138 and the resistance of photocell 28 to reduce extraneous noise pulses thereby eliminating the misfiring of PUT 142.

Resistors 146 and 148 are connected across lines 124 and 126 and function as a voltage divider to provide a reference voltage to the gate of PUT 142. Variable resistor 150 is connected in parallel with resistor 146 and provides a means for adjusting the threshold voltage or firing point of PUT 142.

Connected between the cathode of PUT 142 and line 126 is resistor 152 which functions to limit the anode to cathode current of PUT 142 to a value below the valley current value when it fires.

The current pulse that occurs in resistor 152 generates a voltage across resistor 152 which is coupled, via capacitor 153 to an inverting amplifier transistor state comprised of transistor 154, resistors 156 and 158 and output coupling capacitor 160. Capacitor 160 couples the pulse to a conventional monostable multivibrator circuit comprised of transistors 162 and 164; resistors 166, 168, 170 and 172; and capacitors 174 and 176. Transistor 162 is normally off and transistor 164 is normally on.

The collector electrode of transistor 164 is coupled, via diode 178, to the gate electrode of SCR 104. Since transistor 164 is normally on, no voltage is available at the gate of SCR 104 to turn it on or fire it, so it remains in its non-conducting state.

Resistor 180 is connected from the gate to the cathode of SCR 104 and functions to provide an external DC current path between gate and cathode and reduces the possibility of misfiring of SCR 104 due to noise pulses.

Diode 182 which is connected in series with SCR 104 and solenoid 60 across the full wave bridge circuit 63 helps to insure that SCR 104 turns off after firing, when the rectified full wave voltage reduces to zero. Free wheeling diode 184 is connected across solenoid 60 and functions to maintain current flow in the solenoid for a short time after the SCR 104 turns off.

The operation of the circuit is explained as follows. A shuttle containing a bobbin which has weft material thereon with a diffuse reflecting characteristic reflects light rays to photocells 28 and 30. Since both photocells sense nearly equal light their impedances are nearly the same and the voltage appearing at the anode electrode of PUT 142 is essentially one half of the voltage appearing across lines 124 and 126.

It is to be noted that changes in the ambient light level or changes in the intensity of the light source will effect both photocells equally so that the voltage at the anode electrode of PUT 142 will not change.

The voltage at the anode of PUT 142 V is given by:

and is a function only of the ratio of the resistance of photocells 28 and 30.

When the weft material is exhausted from the specular reflecting surface of the bobbin it permits more light rays to reach photocell 28 than reaches photocell 30. Photocell 28 senses the additional light and changes to its low resistance state. The magnitude of its resistance is a function of the amount of light it receives. The initial light sensed by the photocell causes the resistance to drop fairly rapidly and then recover somewhat to a resistance value slightly larger than its initial resistance when it first sensed light but much lower than its resistance when it is sensing only light reflected from the diffuse reflecting surface. Since the ratio of resistances of photocell 28 to 30 has gone down, the voltage at the anode electrode of PUT 142 increases. When the light is sufficient to cause the anode electrode voltage of PUT 142 to exceed the value previously set on the gate electrode of PUT 142 it will fire, thereby generating a positive pulse across resistor 152 which is inverted and amplified by transistor 154.

The negative pulse at the collector of transistor 154 is coupled via capacitors and 174 to the base electrode of transistor 164 which is driven to an off condition. This permits the DC voltage on line 124 to be coupled through resistor and diode 178 to the gate of SCR 104 thereby firing it. In this condition the monostable multivibrator 136 is in its unstable state and will remain so for a period which is determined by the time constant of resistor 168 and capacitor 174 (R188 m)- When SCR 104 fires, almost all of the voltage across the full wave bridge circuit 63 appears across solenoid 60 energizing it. Solenoid 60 remains in this condition as long as SCR 104 conducts.

A mechanical linkage (not shown) is activated by the energizing of the solenoid 60 which in turn causes the bobbin change mechanism (also not shown) to latch up and operate in a conventional manner.

In a preferred embodiment of the invention the time constant of the monostable multivibrator is approximately 275 milliseconds. At the expiration of the time constant the capacitor 174 is charged up to almost the full DC voltage between lines 124 and 126. The positive voltage on the capacitor 174 turns on transistor 164 again which in turn turns off transistor 162. Thus the monostable multivibrator 136 returns to its normal state and is ready for the next cycle. With the positive voltage withdrawn from the gate of SCR 104 it will turn off as soon as the current flow through it reduces to below its holding circuit. The current through the SCR 104 reduces to substantially zero when the voltage from the full wave bridge circuit 63 reduces to zero each half cycle thereby releasing solenoid 60 until the next time the weft material is exhausted. Thus has been disclosed a new and improved weft detector sensing system which is sensitive and reliable.

It is also to be noted that the present invention may be utilized equally as well if the bobbin has a diffuse reflecting surface and the weft material has a specular reflecting surface with an obvious modification of the present circuitry. The circuitry would be modified in a conventional manner to provide a gating pulse to SCR 104 when the weft material is expanded.

While a preferred embodiment of the invention has been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit and scope of the following claims.

Having thus set forth the nature of this invention, what we claim herein is:

1. A system for detecting in a specified area the presence or absence of material having a first reflecting surface comprising:

a. means having a second reflecting surface for supporting said material;

b. means for providing light to said specified area;

0. first means arranged to sense said light reflected from said specified area in the presence or absence of material, said first means having an impedance proportional to the amount of light sensed;

(1. second means arranged only to sense light reflected from said specified area in the presence of said material, said second means having an impedance proportional to the amount of light sensed;

e. utilization means; and

f. third means coupled between said first and second means and said utilization means for activating said utilization means in accordance with a predetermined ratio of impedances of said first and second means responsively to the presence or absence of said material.

2. A system for detecting in a specified area the presence or absence of material according to claim 1 wherein said first reflecting surface is a diffuse reflecting surface and said second reflecting surface is a specular reflecting surface.

3. A system for detecting in a specified area the presence or absence of material according to claim 1 wherein said first reflecting surface is a specular reflecting surface and said second reflecting surface is a diffuse reflecting surface.

4. A detection system according to claim 1 wherein said light is incident to said material supporting surface,

said first and second light sensing means is a photocell, said first photocell being positioned to receive said reflected light along a path having an angle equal to said incident angle, said second photocell being positioned to receive said reflected light at a distance from said material supporting surface which is substantially equal to the distance said first photocell is from said material supporting surface.

5. A detection system according to claim 4 wherein said first and second photocells are recessedly mounted within a holding frame also containing said light source and being relatively responsive only to light reflected from said specified area.

6. A detection system according to claim 4 including at least a second means for providing light and at least a third means for sensing said light wherein all said light providing means and all said light sensing means cooperatively operate to determine when said utilization means is to be activated.

7. A loom weft detecting system for detecting the exhaustion of weft having a diffuse reflecting surface from a specified area comprising:

a. a bobbin for supporting said weft, said bobbin having a specular reflecting surface relative to that of the weft;

b. means illuminating said specified area by a beam of light incident to said bobbin surface at an angle to the normal to said bobbin surface;

c. a first photocell for receiving specular and diffuse light from said surface area;

d. a second photocell for receiving diffuse reflected light from said surface area being positioned closely adjacent said incident beam of light and at a distance from said surface substantially equal to the distance said first photocell is from said surface;

e. means deriving a DC voltage related to the ratio of the resistance of said first and second photocells;

f. means establishing a threshold voltage, said threshold voltage being intermediate in value between the values of the voltages established by the photocells in the presence and in the absence of said weft on the surface of the illuminated area; and

g. means responsive to the derived DC voltage value exceeding said threshold voltage for actuating a solenoid when said weft is exhausted from said bobbin.

8. A loom weft detecting system for detecting the exhaustion of weft having a specular reflecting surface from a specified area comprising:

a. a bobbin for supporting said weft, said bobbin having a diffuse reflecting surface relative to that of the weft;

b. means illuminating said specified area by a beam of light incident to said bobbin surface at an angle to the normal to said bobbin surface;

c. a first photocell for receiving specular and diffuse light from said surface area;

d. a second photocell for receiving diffuse reflected light from said surface area being positioned closely adjacent said incident beam of light and at a distance from said surface substantially equal to the distance said first photocell is from said surface;

and

g. means responsive to the derived DC voltage value exceeding said threshold voltage for actuating a solenoid when said weft is exhausted from said bobbin. 

1. A system for detecting in a specified area the presence or absence of material having a first reflecting surface comprising: a. means having a second reflecting surface for supporting said material; b. means for providing light to said specified area; c. first means arranged to sense said light reflected from said specified area in the presence or absence of material, said first means having an impedance proportional to the amount of light sensed; d. second means arranged only to sense light reflected from said specified area in the presence of said material, said second means having an impedance proportional to the amount of light sensed; e. utilization means; and f. third means coupled between said first and second means and said utilization means for activating said utilization means in accordance with a predetermined ratio of impedances of said first and second means responsively to the presence or absence of said material.
 2. A system for detecting in a specified area the presence or absence of material according to claim 1 wherein said first reflecting surface is a diffuse reflecting surface and said second reflecting surface is a specular reflecting surface.
 3. A system for detecting in a specified area the presence or absence of material accOrding to claim 1 wherein said first reflecting surface is a specular reflecting surface and said second reflecting surface is a diffuse reflecting surface.
 4. A detection system according to claim 1 wherein said light is incident to said material supporting surface, said first and second light sensing means is a photocell, said first photocell being positioned to receive said reflected light along a path having an angle equal to said incident angle, said second photocell being positioned to receive said reflected light at a distance from said material supporting surface which is substantially equal to the distance said first photocell is from said material supporting surface.
 5. A detection system according to claim 4 wherein said first and second photocells are recessedly mounted within a holding frame also containing said light source and being relatively responsive only to light reflected from said specified area.
 6. A detection system according to claim 4 including at least a second means for providing light and at least a third means for sensing said light wherein all said light providing means and all said light sensing means cooperatively operate to determine when said utilization means is to be activated.
 7. A loom weft detecting system for detecting the exhaustion of weft having a diffuse reflecting surface from a specified area comprising: a. a bobbin for supporting said weft, said bobbin having a specular reflecting surface relative to that of the weft; b. means illuminating said specified area by a beam of light incident to said bobbin surface at an angle to the normal to said bobbin surface; c. a first photocell for receiving specular and diffuse light from said surface area; d. a second photocell for receiving diffuse reflected light from said surface area being positioned closely adjacent said incident beam of light and at a distance from said surface substantially equal to the distance said first photocell is from said surface; e. means deriving a DC voltage related to the ratio of the resistance of said first and second photocells; f. means establishing a threshold voltage, said threshold voltage being intermediate in value between the values of the voltages established by the photocells in the presence and in the absence of said weft on the surface of the illuminated area; and g. means responsive to the derived DC voltage value exceeding said threshold voltage for actuating a solenoid when said weft is exhausted from said bobbin.
 8. A loom weft detecting system for detecting the exhaustion of weft having a specular reflecting surface from a specified area comprising: a. a bobbin for supporting said weft, said bobbin having a diffuse reflecting surface relative to that of the weft; b. means illuminating said specified area by a beam of light incident to said bobbin surface at an angle to the normal to said bobbin surface; c. a first photocell for receiving specular and diffuse light from said surface area; d. a second photocell for receiving diffuse reflected light from said surface area being positioned closely adjacent said incident beam of light and at a distance from said surface substantially equal to the distance said first photocell is from said surface; e. means deriving a DC voltage related to the ratio of the resistance of said first and second photocells; f. means establishing a threshold voltage, said threshold voltage being intermediate in value between the values of the voltages established by the photocells in the presence and in the absence of said weft on the surface of the illuminated area; and g. means responsive to the derived DC voltage value exceeding said threshold voltage for actuating a solenoid when said weft is exhausted from said bobbin. 